The present invention involves a spectral line filtering device and more particularly an optical filter utilizing atomic vapor and the phenomenon of selective specular reflection.
Currently in the optical art there is a need for narrowband filters which can be selectively matched to particular spectral lines since atomic spectral line widths are of the order of 0.01-0.1 Angstroms while commonly available filters, such as interference filters or monochromators, range from 1 to 100 Angstroms in bandwidth.
It has long been known that an atomic vapor will absorb and reradiate incident radiation of a resonant wavelength, but the application of the phenomenon for filtering purposes has only recently been appreciated and utilized in the "volume effect" mode in a filter device disclosed in U.S. Pat. No. 3,504,216. More particularly, such reradiated energy consists of two reflection components. One component is roughly at the same wavelength as the incident energy absorbed and is scattered in all directions so that the vapor functions as a diffuse reflector. This type of reflection is referred to as "volume effect" reflectance since the absorption and reradiation occur within the volume of the vapor. The other reflection component is at the exact wavelength of the absorbed incident energy and arises from interference effects between neighboring atoms. Because the atomic positions in a vapor are random, constructive interference will occur only when the angle of incidence of the energy is equal to the angle of reflection. Given this condition a vapor may function as a specular reflector, that is, in the manner of a mirror, for incident energy of the exact wavelength absorbed. This type of reflectance is referred to as "surface effect" reflectance since the absorption and reradiation occur at the surface region of the vapor.
The relative strengths of the specular and diffuse reflections from any atomic vapor will depend, among other factors, upon the atomic density of the vapor, and the particular spectral line absorbed will depend upon the element vaporized. Specular reflection will predominate in most cases only at pressures of several Torr or greater.
The recently developed prior art vapor filters, such as disclosed in U.S. Pat. No. 3,504,216, have used the "volume effect" mode of atomic vapor reradiation while the "surface effect" mode has generally been ignored and little appreciated. Accordingly, the present invention is intended to provide an optical filter, using the "surface effect," which has a narrow bandwidth in the range of 0.1 Angstrom and yet a comparatively large light acceptance angle and area, and unlike the "volume effect" filters, can be used for both absorption and emission spectrometry.